As set in the Indonesian context of changing policies regarding the language of instruction in a particular stream of public schools, known as the International Standard Schools (ISSs), the study reported in this thesis explored the experiences of Science and Mathematics teachers regarding the introduction, implementation, and reversal of the use of English as the medium of instruction (EMI). Through this study, I aimed to develop insights with regard to teachers' experiences of educational language policy change and reversal. A qualitative, narrative-based approach to the study was adopted. I sought to attend to the meanings that seven Science and Mathematics teachers attributed - through their narration with me - to their experiences of the language of instruction policy change and reversal. A broader context for their meaning-making narratives was gained from the narrativised experiences of one curriculum leader and one teacher trainer. The narrations took place in the participants' preferred language of Bahasa Indonesia with a mixture of some vernacular languages, and, having restoried their narrations into reader friendly texts in the original language(s), I then analysed them from holistic and categorical content perspectives. Next, as informed by this analysis and my reflexively-surfaced understandings of this context, I explored the influences that may have shaped the teachers' experiences. This study identified main patterns in the teacher experiences of change, namely: i) a willingness to embrace change - struggle - fulfilment - disappointment; ii) a willingness to embrace change - excitement - fulfilment - disappointment; and iii) a resistance to change- struggle - fulfilment - relief. There were both internal and external shaping influences on these patterns. Such internal influences include teachers': i) language confidence; ii) perceptions of EMI, and iii) stage of career. These internal influences may have shaped teachers' willingness either to make the most of their involvement in the programme or to simply be part of the programme without many expectations. Possible external shaping influences include: i) societal perceptions of EMI; ii) support from school and government; iii) opportunities to interact with other teachers; and iv) support from colleagues. These external influences seemed to have created an environment which either facilitated or hindered the teachers' performance as EMI teachers and their continuing development for and through the EMI programme. My study also identified three stages of experiences of reversal, i.e.: Stage One - turbulence, intense-emotions, and feelings of nostalgia; Stage Two - readjustments; and Stage Three - acceptance and adaptation. The teachers who responded negatively towards the reversal tended to experience all the three stages. Meanwile, those who were relieved by the reversal directly moved to Stage Three: acceptance and adaptation, without experiencing Stage One and Stage Two. My study has implications for how educational language policy change and reversal may be addressed in countries with a complex linguistic landscape, e.g. Indonesia. It also offers some suggestions for policy makers and teacher educators regarding teachers' experiences and needs when potentially reversing existing or introducing a new educational language policy.